Various methods and devices have been proposed and tried for increasing the penetration rate in drilling. An example of such devices are percussion tools as disclosed in U.S. Pat. Nos. 3,612,191 and 3,768,576 to Martini, inventor of the present invention, U.S. Pat. No. 4,054,180 to Bassinger and U.S. Pat. No. 1,892,517 to Pennington. Prior art percussion tools in general have incorporated complicated valving as well as an oscillating hammer activated by a circulating liquid medium. Due to their complexity and high cost, such tools have not been widely accepted by the drilling industry. Air-driven hammer tools have been used but are not generally useful for standard rotary drilling applications. At present, there is no simple, economical and long-lived percussion-type tool for use in rotary drilling systems.